Conventionally, DSA (Digital Subtraction Angiography) has been conducted using an angiography contrast medium to obtain blood vessel images. DSA adopts an image processing technique that takes difference (subtraction) between image data prior to injecting the contrast medium (hereinafter, referred to as a mask image) and image data subsequent to injecting the contrast medium (hereinafter, referred to as a live image.) The difference between the mask image and the live image can cause elimination of an unnecessary portion for vascular diagnosis for bones or the like from the live image. Thereby a subtraction image containing an image of a blood vessel with the contrast medium injected thereinto needed for diagnosis can be obtained, and thus the blood vessel is readily visible. For conducting DSA satisfactorily, two or more mask images are obtained and they are averaged, whereby a reference mask image is generated. The reference mask image is subtracted from the live image to obtain a desired subtraction image.
When the mask image and the live image are taken with fixed values of tube voltage, tube current, and imaging time in such DSA, brightness of the fluoroscopic image often differs from target brightness depending on size or thickness of a subject. As a result, variation occurs in the brightness of the subtraction image depending on the subject.
However, change of a tube voltage during DSA causes different quality of X-rays. Thus, the tube voltage with a fixed value is desirable.
Moreover, a tube current value is controlled by filament current. Although the filament current is controlled, response to the tube current value in a short time is slow due to thermal inertia of a filament. Accordingly, a tube current value with a fixed value is also desirable. For the reasons as above, it is desirable that the brightness of the fluoroscopic image is controlled through control of the imaging time.
Then, an X-ray apparatus as described in Japanese Patent Publications No. 2003-209747A and 2004-177251 have been designed with AEC (automatic exposure control function.) The AEC has a function of monitoring a dose of X-ray irradiation by a phototimer and intercepting X-ray irradiation upon reaching a target dose. That is, control of the imaging time can achieve control of the dose of X-rays and the brightness of the taken image. In the DSA, it is desirable to take a mask image and a live image at the same brightness. Thus the imaging time is controlled in real time with the AEC upon taking the first mask image to detect an optimal imaging time. Then mask images and the live images from the second are imaged using the detected imaging time. Thus, the imaging time is controlled in real time with the AEC, thereby the imaging time can be obtained for which suitable brightness is achieved.